Electric heater

ABSTRACT

An electric heater includes a. substrate and an inner plane heating element formed on one surface of the substrate. The inner plane heating element includes an inner pattern portion connecting a start point with an end point. The inner pattern portion includes a first track, a second track located outside the first track and spaced part from the first track, a first bridge connecting the first track with one end of the second track, a third track located outside the second track and spaced apart from the second track, and a second bridge connecting the other end of the second track with the third track. A first gap G1 between the first and second tracks is shorter than a second gap G2 between the second and third tracks along a virtual line crossing the first, second and third tracks and closer to the first bridge than the second bridge.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No. 16/540,448, filed on Aug. 14, 2019, which claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean Patent Application No. 10-2018-0097596, filed on Aug. 21, 2018 which are hereby incorporated by reference in their entirety.

BACKGROUND 1. Field of the Invention

The present invention relates to an electric heater applied to a cooking appliance, and more particularly, to an electric heater having a plane heating element capable of preventing dielectric breakdown from occurring due to a potential difference

2. Discussion of the Related Art

In general, a cooking appliance refers to a device for heating and cooking food using gas or electricity. Various products such as a microwave oven using microwaves, an oven using a heater, a gas stove using gas, an electric stove using electricity or a cooktop including a gas stove or an electric stove have come into widespread use.

The gas stove directly generates flame using gas as a heating source, while the electric stove heats a container and food placed on a top plate thereof using electricity.

In the gas stove, heat loss caused by flame is large and contaminants are discharged due to incomplete combustion, thereby polluting room air. Therefore, recently, electric stoves are attracting attention.

Electric stoves may be classified into an inductive electric stove which directly heats a container in which a magnetic field is generated by a magnetic induction method, and a resistive electric stove which heats a top surface made of ceramic using a hot wire.

The inductive electric stove has a short cooking time at a high temperature and must use a dedicated magnetic container. The resistive electric stove may use an existing container but has a relatively long cooking time.

Although an existing resistive electric stove uses a heating element made of a nichrome wire, an electric heater using a plane heating element is being developed in order reduce the thickness of the heating element.

In addition, in order to shorten the cooking time, a resistive electric stove using an electric heater capable of heating a limited area at a high temperature is being developed.

As an example of such an electric heater, Korean Patent Registration No. 10-1762159 B1 (Aug. 4, 2017) discloses a plane heating element including a substrate including a surface made of an electrically insulating material, a heating element attached to the surface of the substrate and having a predetermined shape, and a power supply for supplying electricity to the heating element.

In the electric heater, the temperature distribution of an object to be heated may be changed according to the shape (that is, the pattern) of the plane heating element, and the plane heating element may be formed in a shape capable of heating the object to be heated as uniformly as possible.

The plane heating element of the electric heater includes a plurality of tracks having a straight-line shape or an arc shape and adjacent tracks of the plurality of tracks may be connected through a bridge (or a track).

As another example of the heater, European Patent Publication No. EP 0,228,808A2 (published on Jul. 15, 1987) discloses a temperature sensitive device. Such a device is configured by printing a heater track made of a conductive material and a plurality of electrodes on a ceramic coating layer. As current is supplied through the electrodes, radiant heat is generated in the heater track.

However, a conventional plane heating element includes a heating portion in which one hot wire is formed in a predetermined pattern within a limited area. In order to generate heat at a temperature higher than 500° C., the heating portion is designed to have high resistance.

Accordingly, a large potential difference is generated according to the shape of the pattern or the positions of the electrodes, excessive local heating occurs in a portion in which the potential difference is large and the hot wire is destroyed.

FIGS. 1A and 1B are views showing an example in which dielectric breakdown occurs in a conventional plane heating element.

When alternating current (AC) is used, a capacitive reactance resistance occurs between hot wires. However, since the dielectric constant of a base material increases in. a high-temperature heating environment, capacitive reactance rapidly decreases and local heating and dielectric breakdown occur a zone in which the potential difference large.

When a gap between hot wires of the pattern is uniform according to the prior art, dielectric breakdown occurs between outermost bridges of the pattern A as shown in FIG. 1 a or between the electrodes B as shown in FIG. 1 b.

In order to configure a high-temperature plane heating element formed in the limited area, there is a to design a pattern and electrodes capable of solving local heating and dielectric breakdown occurring due to a potential difference.

SUMMARY

An object of the present invention is to provide electric heater including a plane heating element capable of solving local heating and dielectric breakdown even when a pattern portion is configured within a limited area.

Another object of the present invention is to provide an electric heater including plane heating element capable of uniformly generating heat by a pattern portion.

Another object of the present invention is to provide an electric heater including a plane heating element capable of preventing dielectric breakdown generated in a portion of an electrode portion and a pattern portion.

To achieve the above objects, there is provided an electric heater including a substrate (an insulating material capable of forming a conductor pattern on a surface of an. insulating substrate) and an inner plane heating element formed on one surface of the substrate, wherein the inner plane heating element includes an inner pattern portion connecting a start point with an end point, wherein the inner pattern portion includes a first track, a second track located outside the first track and spaced part from the first track, a first bridge connecting the first track with one end of the second track, a third track located outside the second track and spaced apart from the second track, and a second bridge connecting the other end of the second track with the third track, and wherein a first gap G1 between the first track and the second track is shorter than a second gap G2 between the second track and the third track along a virtual line crossing the first track, the second track and the third track and closer to the first bridge than the second bridge. At this time, each of the first, second and third tracks may have an arc shape and the virtual line may pass through one of arc centers of the first, second and third tracks.

Accordingly, the gap between the tracks of the pattern portion having a smallest potential difference within the limited area is shortest and the gap between the tracks of the pattern portion having a large potential difference is relatively large, thereby solving local heating and dielectric breakdown in the pattern portion.

In the present invention, the inner plane heating element may include a pair of inner electrodes connected to the inner pattern portion, and a third gap G3 between the inner electrodes may be 12 mm or more. That the gap between the electrodes having a large potential difference is large, thereby preventing dielectric breakdown of the electrodes.

In the present invention, the inner pattern portion includes a plurality of arc-shaped inner tracks spaced apart from each other and each having a length increasing from an inner side to an outer side and a plurality of inner bridges connecting the inner tracks in series, the inner electrodes may be connected to an outermost side of the inner pattern portion, and the inner tracks, the inner bridges and the inner electrodes may be symmetrical with respect to a reference line passing through a center of the inner pattern portion. That is, the pattern portion for symmetrically generating heat is configured, by uniformly generating heat in the entire zone.

In the present invention, a gap G4 between the inner bridges facing each other with respect to the reference line may be shortest at an innermost side of the inner pattern portion or may be shorter than a gap G3 between the inner electrodes. That is, as the potential difference decreases, the gap between the bridges decreases, thereby designing the bridges in the limited area in consideration of the potential difference.

In the present invention, a gap G4 between the inner bridges facing each other with respect to the reference line may be longer than the first gap G1 and may be 6 mm or more. That is the gap between the bridges increases, thereby preventing the dielectric breakdown of the bridges and local heating.

In the present invention, a thickness T₁ of each of the inner electrodes may be at least 1.5 times greater than a thickness T₀ of each of the inner tracks and a thickness T₂ of each of the inner bridges may be at least 1.5 times greater than a thickness T₀ of each of the inner tracks. That is, the resistance of the electrode or the bridge having a relatively large potential difference decreases, thereby efficiently decreasing a heating temperature.

In the present invention, the electric heater further includes an outer plane heating element located outside the inner plane heating element and formed on one surface of the substrate, and the outer plane heating element may include at least one outer pattern portion located outside the inner pattern portion. and connecting a start point with an end point, and a pair of outer electrodes connected to the outer pattern portion. That is, it is possible to configure multiple pattern portions.

In the present invention, the outer electrodes may horizontally located outside the inner electrodes.

In addition, the outer pattern portion may include a plurality of arc-shaped outer tracks spaced apart from each other and each having a length increasing from an inner side to an outer side and a plurality of outer bridges connecting the outer tracks in series, the outer electrodes may be connected to an innermost side of the outer pattern portion, and the outer tracks, the outer bridges and the outer electrodes may be symmetrical with respect to a reference line passing through a center of the outer pattern portion.

In particular, at least some of the inner/outer electrodes may be located between a pair of outer bridges.

In addition, a thickness T₃ of each of the outer electrodes may be at least 1.5 times greater than a thickness T₀ of each of the outer tracks, and a thickness T₄ of each of the outer bridges may be at least 1.5 times greater than a thickness T₀ of each of the outer tracks. That is, the resistance of the electrode or the bridge having a relatively large potential difference decreases, thereby efficiently decreasing a heating temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing or photograph executed in color. Copies of this patent or patent application publication with color drawing(s) or photograph(s) will be provided by the Office upon request and payment of the necessary fee.

FIGS. 1A and 1B are views showing an example is which dielectric breakdown occurs in a pattern and an electrode of a conventional plane heating element.

FIG. 2 is a perspective view showing an electric stove, to which as electric heater according to an embodiment of the present invention is applied.

FIG. 3 is a control block diagram of an electric stove, to which an electric heater according to an embodiment of the present invention is applied.

FIG. 4 is a cross-sectional view showing an electric heater according to an embodiment of the present invention.

FIG. 5 is a plan view showing a single pattern type plane heating element according to an embodiment the present invention.

FIG. 6 is a view showing a portion of a pattern portion applied to FIG. 5 .

FIGS. 7 to 8 are cross-sectional views taken along lines A-A′ and B-B′ of FIG. 5

FIG. 9 is a plan view showing a single pattern type plan heating element according to an embodiment of the present invention.

FIGS. 10 to 11 are cross-sectional views taken along lines C-C′ and D-D′ of FIG. 9 .

FIGS. 12A and 12B are views showing heating states before/after the electrode and bridge of a plane heating element is over-coated according to the present invention.

DETAILED DESCRIPTION

Hereinafter, the present embodiment will be described in detail with reference to the accompanying drawings. It should be understood, however, that the scope of the inventive concept of the present embodiment may be determined from the matters disclosed in the present embodiment, and the spirit of the present invention possessed the present embodiment includes implementations such as addition, deletion, modification etc. of components to the proposed embodiment.

FIG. 2 is a perspective view showing an electric stove, to which an electric heater according to an embodiment of the present invention applied, and FIG. 3 is a control block diagram of an electric stove, to which an electric heater according to an embodiment of the present invention is applied.

The electric heater 1 of the present invention may configure a portion of an electric stove such as a cooktop.

The electric may include a cabinet 2 forming appearance. The electric heater 1 may be provided on the cabinet 2. The upper surface of the cabinet 2 may be opened and the electric heater 1 may be provided on the upper surface of the cabinet 2.

The electric stove may include an input unit 3 for manipulating the electric stove and a display 4 for displaying a variety of information such as information on the electric stove. In addition, the electric stove may further include a power supply 5 connected to the electric heater 1 to apply current to the electric heater 1. The electric stove may further include a controller 6 for controlling the power supply 5 and the display 4 according to input of the input unit 3.

The electric heater 1 may be provided on the cabinet 2 such that the upper surface thereof is exposed to the outside. An object to be heated by the electric stove may be placed on the upper surface of the electric heater 1, and the upper surface of the electric heater 1 may be a surface in which the object to be heated is seated.

FIG. 4 is a cross-sectional view showing an electric heater according to an embodiment of the present invention.

The electric heater 1 may include a substrate 10 and a plurality of plane heating element 100 formed on one surface of the substrate 10.

The substrate 10 may be an insulating substrate having a conductor pattern formed on a surface thereof. The upper surface of the substrate 10 may be a surface 13 in which the object to be heated is seated. The lower surface of the substrate 10 may be a surface 14 in which the plane heating element 100 are formed.

The substrate 10 may include only a base 11 formed of an insulating material or may include a base 11 formed of an insulating material or a non-insulating material and an insulating layer 12 formed on one surface of the base 11.

The base 11 may be glass and the insulating layer 12 may be formed on the lower surface of the glass using a coating or a printing method.

The plane heating element 100 may be directly formed on one surface of the base 11 formed of an insulating material or may be formed on the insulating layer 12.

The base 11 may be formed in a shape of a plate on which the object to be heated is placed or in a shape of a container in which the object to be heated is received.

The insulating layer 12 may be formed on the lower surface of the base 11. The insulating layer 12 may be formed on the entire lower surface of the base 11 or may be formed on a portion of the lower surface of the base 11. The insulating layer 12 may be formed only in a zone in which the plane heating element 100 will be formed. The insulating layer 12 may configure the entire lower surface of the substrate 10 or a portion of the lower surface of the substrate 10.

The plane heating element 100 may be formed on the lower surface 14 of the insulating layer 12. The plane heating element 100 may have a size smaller than the substrate 10 and the lower surface of the substrate 10 may have a heated zone H, in which the plane heating element 100 is formed, and an unheated zone UH located around the heated zone H.

The heater 1 may further include a coating layer 18 surrounding the plane heating element 100. The coating layer 18 may be formed of an electrically insulating material to protect the plane heating element 100.

The substrate 10 of the present embodiment may be formed of a flexible material, such as a flexible insulating film. In this case, the electric heater 1 may be a flexible planar heater. Such a flexible planar heater may be attached to a member, on which the object to be heated is placed, to heat the object to be heated, like the upper plate of the electric stove.

FIGS. 5 to 8 are views showing a single pattern type plane heating element according to an embodiment of the present invention.

The single pattern type plane heating element according to the present invention may include only a first plane heating element 100 composed of one film-type heater track as shown in FIGS. 5 to 8 , and include a pattern portion 110, in which the film-type heater track is arranged in a predetermined shape, and an electrode portion 120 for supplying current to the pattern portion 110.

The pattern portion 110 includes a heating portion closely arranged in a limited circular area. The pattern portion is configured such that a start point and an end point located at outermost sides are connected along various paths, and is bilaterally symmetrical with respect to the center O of the pattern portion 110 shown in FIG. 5 .

According to the embodiment, the pattern portion 110 may include a plurality of arc-shaped tracks 111 spaced apart from each other and having a length increasing from the center to an outer side and a plurality of bridges 112 connecting the tracks 111 in series.

At this time, the area of the pattern portion 110 and the length of the film-type heater track configuring the pattern portion 110 may be set to be proportional to required power.

The electrodes 120 are composed of a non-heating portion having a relatively lower heat value than the pattern portion 110 and include a positive electrode 121 for receiving current and a negative electrode 122 for outputting current. The positive electrode 121 and the negative electrode 122 may be horizontally located at a predetermined gap.

The positive electrode 121 is a portion extending from the start point of the pattern portion 110 and connected to an external input terminal, and the negative electrode 122 is a portion extending from the end point of the pattern portion 110 and connected to an external output terminal.

When current is supplied to the single pattern type plane heating element, th pattern portion 110 acts as a resistor and heat is generated in a bilaterally symmetrical form.

While current flows, a voltage is gradually decreased from the start point to the end point of the pattern portion 110 according to position, and a potential difference occurs between the tracks 111 and the bridges 112 located closely.

A large potential difference is generated between the positive electrode 121 and the negative electrode 122. In order to maintain an insulation gap, a third gap G3 between the positive electrode 121 and the negative electrode 122 may be longest.

According to the embodiment, the third gap G3 may be 12 mm or more, in order to prevent dielectric breakdown between the positive electrode 121 and the negative electrode 122.

The start point and the end point of the pattern portion 110 are located at the outermost side of the pattern portion 110, and the tracks 111 and the bridges 112 may be formed to be bilaterally symmetrical with respect to the center of the pattern portion 110.

When current flows along the pattern portion 110, the voltage is decreased from the outside to the inside of the left bridges 112 and the voltage is decreased from the inside to the outside of the right bridges 112. That is, smallest potential difference is generated between the bridges 112 located at the innermost side of the pattern portion 110, and a smallest insulation gap may be formed.

Accordingly, a fourth gap G4 between the bridges 112 facing each other with respect to the reference line may be shortest.

At this time, in order to prevent dielectric breakdown of the bridges 112, the fourth gap G4 may be 6 mm or more to secure an insulation gap.

In a portion of the pattern portion 110, as shown in FIG. 6 , a second track 111 b is located outside a first track 111 a, a third track 111 c is located outside the second track 111 b, a first bridge 112 a connecting the first track 111 a with one end of the second track 111 b is located, and a second bridge 112 b connecting the other end of the second track 111 b with the third track 111 c is located.

When current is supplied, current flows along the first track 111 a, the first bridge 112 a, the second track 111 b, the second bridge 112 b and the third track 111 c, and the voltage is decreased in a direction in which current flows.

At this time, the potential difference between the first track 111 a and the second track 111 b increases as a distance from the first bridge 1112 a increases and the potential difference between the second track 111 b and the third track 111 c increases as a distance from the second bridge 112 b increases.

According, if it is assumed that a virtual line L1 crossing the first track 111 a, the second track 111 b and the third track 111 c and is closer to the first bridge 112 a than the second bridge 112 b is located, a first gap G1 between the first track 111 a and the second track 111 b located on the virtual line L1 is shorter than a second gap G2 between the second rack 111 b and the third track 111 c (G1<G2).

That is, in a zone closer to the first bridge 112 a than the second bridge 112 b with respect to the center line L2 crossing the center O of the first track 111 a, the second track 111 b and the third track 111 c, the first gap G1 is short than the second gap G2.

Of course, the first track 111 a, the second track 111 b and the third track 111 c are spaced apart from one another in an arc shape and the centers of the arcs forming the first track 111 a, the second track 111 b and the third track 111 c may not match.

However, the virtual line may be defined to pass through the center of the arc of at least one of the first track 111 a, the second track 111 b and the third track 111 c and to cross the first track 111 a, the second track 111 b and third track 111 c.

In order to reduce the potential difference, in a portion connected to the bridge, the first gap G1 between the tracks is shortest, the third gap G3 between the electrodes is longest, and the fourth gap G4 between the bridges facing each other with respect to the reference line is longer than the first gap G1 and is shorter than the third gap G3 (G1<G4<G3).

In addition, in order to prevent dielectric breakdown in a high-temperature environment, the third gap G3 between the electrodes may be 12 mm or more and the fourth gap G4 between the bridges may be 6 mm or more, thereby securing an insulation gap.

However, in order to form the pattern portion and the electrodes within the limited area, the third gap G3 or the fourth gap G4 may be limited.

Accordingly, if resistance of the electrodes or bridges having a relatively large potential difference is somewhat lowered, it is possible to reduce a heating temperature at that point and to prevent dielectric breakdown.

According to the embodiment, as shown in FIG. 8 , the thickness T₁ or the positive electrode 121 may be at least 1.5 times greater than the thickness T₀ of the track 111 on the pattern portion side and, as shown in FIG. 6 , the thickness T₂ of the bridge 112 on the pattern portion side may be at least 1.5 times greater than the thickness T₀ of the track 111 on the pattern portion side.

That is, the pattern portion 110 and the electrode portion are printed with the same thickness T₀, and then over-coating may be performed at the positions of the electrode portion 120 and the bridge 112 with predetermined thickness T₁-T₀ and T₂-T₀ or printing may be performed at least twice, without being limited thereto.

FIGS. 9 to 11 are views showing a single pattern type plan heating element according to an embodiment of the present invention.

The single pattern type plane heating element according to the present invention includes an inner plane heating element 100 located at the center of the same plane and an outer plane heating element 200 located to surround the inner plane heating element 100, as shown in FIGS. 9 to 11 .

The inner plane heating element 100 includes an inner pattern portion 110 arranged in a predetermined shape and an inner electrode portion 120 connected to the inner pattern portion 110, and has the same configuration as the above-described single pattern type plane heating element. Therefore, a detailed description thereof will be omitted.

The outer plan heating element 200 includes an outer pattern portion 210 located outside the inner pattern portion 110 and arranged in a predetermined shape, and an outer electrode portion 220 connected to the outer pattern portion 210.

The outer pattern portion 210 includes a heating portion closely arranged in limited area having a ring shape and surrounding the outside of the inner pattern portion 110. The outer pattern portion is configured such that a start point and an end point located at innermost sides are connected along various paths, and is bilaterally symmetrical.

According to the embodiment, the outer pattern portion 210 may include a plurality of outer tracks 211 and a plurality of outer bridges 212, similarly to the inner pattern portion 110.

In addition, a portion of the inner electrode portion 120 and a portion of the outer electrode portion 220, which will be described blow, may be located between. the outer bridges 212.

According, at least the inner and outer electrode portions 120 and 220 having a relatively lower heating temperature than the inner and outer pattern portions 110 and 210 are included between the outer bridges 220, thereby increasing a hot zone in which heat is generated at 500° C. or more and decreasing a dead zone in which heat is generated at a temperature less than the hot zone by 300° C. or less in the heated zone.

Of course, the inner and outer pattern portions 110 and 210 may be designed to distribute required power and the areas of the pattern portions 110 and 210 and the lengths of the film-type heater tracks configuring the pattern portions 110 and 210 may be set to be proportional to distributed power.

The outer electrode 220 also includes a non-heating portion having a lower heat value than the outer pattern portion 210, includes an outer positive electrode 221 and an outer negative electrode 222 similarly to the inner electrode portion 120, and is horizontally located outside the inner electrode portion 120.

Similarly, the outer positive electrode 221 is a portion extending from the start point of the outer pattern portion 210 and is connected to an external input terminal, and the outer negative electrode 222 is a portion extending from the end point of the outer pattern portion 210 and is connected to an external output terminal.

At this time, an insulation gap between the positive electrodes 121 and 221 should be maintained and the negative electrodes 122 and 222 may be connected to each other to be grounded.

According to the embodiment, as shown in FIG. 10 , the thickness T₃ of the outer positive electrode 221 may be at least 1.5 times greater than the thickness T₀ of the outer track 211 on the outer pattern portion side and, as shown in FIG. 11 , the thickness T₄ of the outer bridge 212 on the outer pattern portion side may be at least 1.5 times greater than the thickness T₀ of the outer track 211 on the outer pattern portion side.

That is, the outer pattern portion 210 and the outer electrode portion 220 are coated with the same thickness T₀, and then over-coating may be performed at the positions of the outer electrode 220 and the outer bridge 212 with predetermined thickness T₃-T₀ and T₄-T₀ or printing may be performed at least twice, without being limited thereto.

When current is supplied to the dual pattern type plane heating element, the inner pattern portion 110 and the outer pattern portion 210 act as a resistor and heat is generated in a bilaterally symmetrical form.

The inner pattern portion 110 and the inner electrode portion 120 are configured such that a gap therebetween is short at a point having a low potential difference and is long at a point having a high potential difference, thereby preventing local heating and dielectric breakdown in a high-temperature environment even if the potential difference is high.

In addition, the inner and outer electrode portions 120 and 220 and the inner and outer bridges 112 and 212 have relatively larger thicknesses than the inner and outer tracks 111 and 211, thereby decreasing resistance, decreasing a heating temperature at that time, and efficiently preventing dielectric breakdown.

FIGS. 12A and 12B are views showing heating states before/after the electrode and bridge of a plane heating element is over-coated according to the present invention.

According to the present invention, as shown in FIG. 12A, if the gaps between the electrodes and the gaps between the bridges are increased as compared to the prior art, dielectric breakdown between the electrodes and the bridges is solved. However, if the electrodes and the bridges are not relatively thick, relative local heating is maintained.

Accordingly, as shown in FIG. 12B, if the gaps between the electrodes and the gaps between the bridges are increased as compared to the prior art and the electrodes and the bridges are relative thick, it is possible to simultaneously solve dielectric breakdown between the electrodes and the bridges and relative local heating and to heat the entire zone with the uniform temperature.

According to the electric heater of the present invention, the gap between film-type heater tracks having a smallest potential difference in the pattern portion may be shortest and a gap between film-type heater tracks having a large potential difference in the pattern portion may be relatively large.

Accordingly, even if the pattern portion is configured in a limited area in a high-temperature environment, it is possible to solve local heating in the pattern portion through a space, to prevent dielectric breakdown from occurring by local heating in the pattern portion, to uniformly generate heat over the entire area in which the pattern portion is formed, and to secure a maximum heating area within the limited area in which the pattern portion may be formed.

In addition, the gap between the electrodes having a larges potential difference is 12 mm or more or the gap between the innermost bridges having a smallest potential difference in the pattern portion is 6 mm or more, thereby preventing dielectric breakdown of the electrodes and the bridges.

In addition, the electrodes or the bridges of the pattern portion are thicker than the tracks of the pattern portion, thereby efficiently decreasing the heating temperature of the electrode and the bridges which may excessively generate heat. 

What is claimed is:
 1. An electric heater comprising: a substrate; and a plane heating element located at a surface of the substrate and defines a part of a circular area having a center, wherein the plane heating element comprises a pattern portion comprising: a plurality tracks that define arc shapes about the center and are spaced apart from one another in a radial direction of the circular area, wherein a length of one of the plurality of tracks arranged at an outer side is greater than a length of another of the plurality of tracks arranged at an inner side relative to the outer side in the radial direction, a plurality of bridges that connect first and second tracks among the plurality of tracks in series, the plurality of bridges comprising a first bridge that connects the first track to one end of the second track in series, and a pair of electrodes that are connected to the pattern portion and define an electrode gap therebetween, wherein each of the plurality of tracks, the plurality of bridges, and the pair of electrodes is symmetrical with respect to a reference line that passes through the center, and wherein the pattern portion defines a gap between any two bridges among the plurality of bridges that face each other with respect to the reference line, the gap being narrower than the electrode gap.
 2. The electric heater of claim 1, wherein the pair of electrodes are connected to outermost tracks that are located at farthest positions from the center among the plurality of tracks in the radial direction away from the center.
 3. The electric heater of claim 1, wherein a minimum of the gaps is defined between innermost tracks that are disposed at closest portions to the center among the plurality of tracks.
 4. The electric heater of claim 1, wherein a thickness of each of the pair of electrodes is greater than a thickness of each of the plurality of tracks in a direction perpendicular to the substrate.
 5. The electric heater of claim 1, wherein a thickness of each of the plurality of bridges is greater than a thickness of each of the plurality of tracks in a direction perpendicular to the substrate.
 6. The electric heater of claim 1, wherein gap between two bridges facing each other with respect to the reference line and disposed at an innermost side of the pattern portion is shorter than or equal to any one of gaps between other two bridges facing each other with respect to the reference line.
 7. The electric heater of claim 1, wherein the plurality of tracks comprise: a first track; a second track spaced apart from the first track in the radial direction away from the center; and a third track spaced apart from the second track in the radial direction away from the center, wherein the plurality of bridges comprise: the first bridge that connects the first track to a first end of the second track, and a second bridge that connects a second end of the second track to the third track, and wherein the pattern portion defines a first gap between the first track and the second track, and a second gap between the second track and the third track, the second gap being wider than the first gap along a virtual line that crosses the first track, the second track, and the third track and that extends to a portion of the pattern portion closer to the first bridge than to the second bridge.
 8. The electric heater of claim 1, further comprising an outer plane heating element that is located at the surface of the substrate and that is spaced apart from the plane heating element in the radial direction away from the center, wherein the outer plane heating element comprises: at least one outer pattern portion that is spaced apart from the pattern portion in the radial direction away from the center; and a pair of outer electrodes connected to the outer pattern portion.
 9. The electric heater of claim 8, wherein the outer pattern portion comprises: a plurality of outer tracks that define arc shapes about the center and are spaced apart from one another in the radial direction, the plurality of outer tracks being arranged at an inner side and an outer side farther from the center than the inner side in the radial direction, and a plurality of outer bridges that connect the plurality of outer tracks in series, wherein the rat of outer electrodes are connected to innermost outer tracks.
 10. The electric heater of claim 9, wherein each of the plurality of outer tracks, the plurality of outer bridges, and the pair of outer electrodes is symmetrical with respect to the reference line.
 11. The electric heater of claim 10, wherein the pair of outer electrodes is located between a pair of outer bridges among the plurality of outer bridges.
 12. The electric heater of claim 9, wherein a thickness of each of the pair of outer electrodes is greater than a thickness of each of the plurality of outer tracks in a direction perpendicular to the substrate.
 13. The electric heater of claim 9, wherein a thickness of each of the plurality of outer bridges is greater than a thickness of each of the plurality of outer tracks in a direction perpendicular to the substrate.
 14. The electric heater of claim 9, wherein the pair of electrodes are located between the pair of outer electrodes.
 15. The electric heater of claim 7, wherein the first gap decreasing toward the first bridge. 